@interface LDPerson : NSObject
{
    int _no;
    int _age;
}
@end

LDPerson底层实现:

struct NSObject_IMPL{
    Class isa;
}
struct LDPerson_IMPL
{
    struct NSObject_IMPL NSObject_IVARS;//8字节
    int _no;//4字节
    int _age;//4字节
}

所以上图中LDPerson的instance对象占用16字节的内存.

思考下图中LDPerson的instance对象占用的内存大小

@interface LDPerson : NSObject
{
    int _age;
    int _height;
    int _no;
}

底层实现:

struct NSObject_IMPL{
    Class isa;
}
struct LDPerson_IMPL
{
    struct NSObject_IMPL NSObject_IVARS;//8字节
    int _no;//4字节
    int _height;//4字节
    int _age;//4字节
}

推断:实际占用20字节,因为内存对其原则,必须为最大成员变量 所占内存的整数倍.所以该instance对象应该占用24个字节.

通过代码验证:

image.png

通过内存图分析:

image.png

通过OC代码转成C/C++代码再次查看

1. cd 目标路径
2.xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m -o main-arm64.cpp

查看后得知LDPerson底层实现结构就如我们上面所写,所以LDPerson确实占用24字节,分配32字节.

查看objc4开源代码

1. 检索allocWithZone 查看.mm文件

id
_objc_rootAllocWithZone(Class cls, malloc_zone_t *zone)
{
    id obj;

#if __OBJC2__
    // allocWithZone under __OBJC2__ ignores the zone parameter
    (void)zone;
    obj = class_createInstance(cls, 0);
#else
    if (!zone) {
        obj = class_createInstance(cls, 0);
    }
    else {
        obj = class_createInstanceFromZone(cls, 0, zone);
    }
#endif

    if (slowpath(!obj)) obj = callBadAllocHandler(cls);
    return obj;
}

主要代码为:

image.png

id 
class_createInstance(Class cls, size_t extraBytes)
{
    return _class_createInstanceFromZone(cls, extraBytes, nil);
}

可以看到extraBytes传入的值为0.再看_class_createInstanceFromZone的实现:

id
_class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone, 
                              bool cxxConstruct = true, 
                              size_t *outAllocatedSize = nil)
{
    if (!cls) return nil;

    assert(cls->isRealized());

    // Read class's info bits all at once for performance
    bool hasCxxCtor = cls->hasCxxCtor();
    bool hasCxxDtor = cls->hasCxxDtor();
    bool fast = cls->canAllocNonpointer();

    size_t size = cls->instanceSize(extraBytes);
    if (outAllocatedSize) *outAllocatedSize = size;

    id obj;
    if (!zone  &&  fast) {
        obj = (id)calloc(1, size);
        if (!obj) return nil;
        obj->initInstanceIsa(cls, hasCxxDtor);
    } 
    else {
        if (zone) {
            obj = (id)malloc_zone_calloc ((malloc_zone_t *)zone, 1, size);
        } else {
            obj = (id)calloc(1, size);
        }
        if (!obj) return nil;

        // Use raw pointer isa on the assumption that they might be 
        // doing something weird with the zone or RR.
        obj->initIsa(cls);
    }

    if (cxxConstruct && hasCxxCtor) {
        obj = _objc_constructOrFree(obj, cls);
    }

    return obj;
}

image.png

size_t instanceSize(size_t extraBytes) {
        size_t size = alignedInstanceSize() + extraBytes;
        // CF requires all objects be at least 16 bytes.
        if (size < 16) size = 16;
        return size;
    }
 // May be unaligned depending on class's ivars.
    uint32_t unalignedInstanceSize() {
        assert(isRealized());
        return data()->ro->instanceSize;
    }

    // Class's ivar size rounded up to a pointer-size boundary.
    uint32_t alignedInstanceSize() {
        return word_align(unalignedInstanceSize());
    }

对比size_t instanceSize(size_t extraBytes)和class_getInstanceSize(Class cls)的实现

size_t class_getInstanceSize(Class cls)
{
    if (!cls) return 0;
    return cls->alignedInstanceSize();
}

因为extraBytes为0所以两个函数的返回值类似,只是前者多了一个与16字节的判断.
然后我们再看obj = (id)calloc(1, size);这个函数,如下可知C语言的标准库,看不到实现的代码.

image.png

通过其他开源代码查看calloc函数的实现

1. 打开开源代码
2. 检索libmalloc关键字

3.libmalloc-140.40.1.tar.gz
4.按如下所示查找到calloc实现如下:

image.png

iOS堆内存 分配原则:都是16的倍数

可通过下面的方式查看:

image.png